The cutting insert for grooving is required to permit a smooth discharge of chips generated. In order to improve the chip discharge property, various considerations have been conventionally given to the shape of the rake face of the cutting insert (for example, refer to Patent Document 1).
FIG. 21 is a partially enlarged schematic perspective view showing the neighborhood of one end of the related art cutting insert as described in the patent document 1. As shown in FIG. 21, a cutting insert 100 in the related art has, on the upper surface of the insert main body, a rake face 101, a clamp face 102 and an ascending portion (a descending portion) 103 formed into a slope shape from the rake face 101 toward the clamp face 102. A pair of first projections 105 and 105, which are arranged near a front cutting edge 104 and arranged laterally when viewed from the front cutting edge 104, are located on the rake face 101, and a second projection 106 is located posterior to the first projections 105 and 105 on the rear of the rake face 101.
When the grooving is carried out by using the cutting insert 100, firstly, the generated chips are previously curled by the first projections 105 and the second projection 106, and brought into a further dense coil shape and then broken and cut at the ascending portion 103. Thus, the chips caught by the cutting insert and the holder can be reduced to achieve smooth cutting.
Here, the sloped surface constituting the ascending portion 103 is configured so that the chips contact the ascending portion 103 and are curled upward and discharged. This enables effective chip breaking and cutting.
However, there has been a recent increase in materials having excellent ductility as a work material. When such a work material is subjected to the grooving using the cutting insert 100, the chips are hard to be broken and cut. Consequently, there has been the tendency that the curled chips are not discharged and liable to remain on the ascending portion 103.
Therefore, when the cutting insert 100 is used for internal turning in which for example, the inner surface of a tubular work material is subjected to grooving, the machining is carried out with the space over the rake face 101 closed. Hence the remaining chips will remain between the cutting insert 100 and the work material, so that the machined surface might be damaged and the cutting insert 100 might be damaged. When the chip curling on the rake face 101 is reduced to avoid the chips remaining at the ascending portion 103, the chips in the extended state graze the ascending portion 103. As a result, the chips are caught by the cutting insert 100 and the holder, so that the machined surface might be damaged and the cutting insert 100 and the holder might be damaged.
On the other hand, Patent Document 2 and Patent Document 3 disclose cutting inserts in which three types of projections having different distance from the cutting edge are arranged on the rake face. Specifically, in the cutting insert of Patent Document 2, the three types of projections are located so that their respective top portions are linearly arranged in the side view. In the cutting insert of Patent Document 3, the three types of projections are located so that the top portion of the projection having the second largest distance from the cutting edge is the highest among the top portions of the projections in the side view.
However, when the cutting insert of Patent Document 2 is used to perform the grooving of the above-mentioned work material having excellent ductility, the chips may slidingly pass through the top portions of the linearly arranged projections, and they ride on the ascending portion without being curled. When the cutting insert of Patent Document 3 is used to perform the grooving of the above-mentioned work material having excellent ductility, the chips may ride on the ascending portion without contacting the projection having the third largest distance from the cutting edge.
Consequently, in the cutting inserts disclosed in Patent Documents 2 and 3, the chips riding on the ascending portion may be caught by the cutting insert and the holder, thus damaging the machined surface.    Patent Document 1: Japanese Unexamined Utility Model Publication No. 64-56904    Patent Document 2: Japanese Unexamined Patent Publication No. 2000-176708    Patent Document 3: Japanese Unexamined Patent Publication No. 2006-150584